however in this case right now kakashi has never had a big drawn out battle with him he only fought for like a second back in the original naruto series and that was just him copying kisames technique and remember guy just used the eight gates which kakashi would definitely know by now since hes guys "rival" and hes also seen lee go up to five with his sharingan and hes been seen using the first gate on two separate occasions and knows a lot about the eight gates
i know hes used the first gate to climb rocks when was the 2nd time?
and guy without any gates is intensely strong compared to average ninja seeing as he only does taijutsu (so while kakashi can mold lightning and fire and use the sharingan and literally thousands of attacks guy has been working out and practicing fighting hand to hand), so even if he could open all 8 gates i don't think any taijutsu he has copied from guy could stop kisame, who survived guy's ultimate taijutsu attack which was called a instant one hit kill move and had enough stamina to break out of chakra enchanced wood afterwards.
or he could open all 7 gates(assuming he can) and use a ninjutsu, which would probably get sucked up by kisame's water shark missle which eats chakra and adds its power into its own and blasts the enemy which would have happened if guy used a ninjutsu instead.
now kakashi's genjutsu idk if samehada can act as a partner and break genjutsu or not, but he can send chakra into kisame to heal him and is self aware so im guessing it can, so kakashi weather he knows this or not could trap him in one, try to use lightning blade thru his heart and get shaved by samehada and his chakra drained
if kakashi would win id guess it'd be similer to when he beat zabuza the first time, suprise him with a counter ninjutsu early on. but zabuza didn't know how sharingan works, he thought kakashi could see into the future but kisame was partners and friends with one of the most powerful sharingan users in history for years so he probably knows it copies attacks, and if it turns into a war of ranged ninjutsu kakashi is boned since kisame is very very hard to kill, and has tailed beast level chakra and can heal from fatel wounds
My guess, kamui cathing kisame's head and killing him instantly would be the only way he wins, but thats not very fun to watch now is it lol